Previous studies have indicated that antigen-induced breakdown of membrane phospholipids in basophilic RBL-2H3 cells, requires recruitment of the tyrosine kinases, lyn and syk. Other stimulants, for example, adenosine analogs or carbachol, in RBL-2H3 cells transfected with the gene for the muscarinic ml receptors, operate exclusively through G- proteins to activate phospholipase C and other phospholipases. The ensuing events, however, such as mobilization of intra- and extra- cellular Ca2+, as well as the activation of protein kinase C and MAP kinases appear to be identical for all stimulants. Present studies have focussed on events leading to the activation of MAP kinase. All stimulants (antigen, carbachol, thapsigargin and the combination of Ca2+ ionophore and phorbol 12-myristate 13-acetate) activated a G protein (probably ras), raf 1, MEK and p42 mapk. This pathway was inhibited by quercetin and GDP~S. Early events preceding this pathway, however, differed for the various stimulants. For example, the early events following antigen-stimulation were Ca2+-independent while those for carbachol were Ca2+-dependent. The functional significance of the various stimulatory events have been substantially elucidated. Release of secretory granules was dependent exclusively on a modest increase in cytosolic Ca2+ and activation of protein kinase C (~ and isozymes). Release of arachidonic acid was dependent on a Ca2+-induced translocation of a cytosolic phospholipase A2 to membranes and the phosphorylation of this phospholipase by MAP kinase. The regulation of cytokine production and release is the subject of another report (Z01 HL 00990-08 LCP).